feat(kit): add safe background service worker updates

[rdlabo]

Summary

• add an opt-in provideKitAppUpdate({ strategy: 'background' }) mode without changing the existing blocking default
• check for complete Angular service-worker versions without blocking bootstrap, and reload only before the first render completes
• recover unrecoverable startup generations once while preserving history state and preventing reload loops
• document the new app-update entry point and strategy

Safety

• never calls activateUpdate(), avoiding mixed application generations
• never reloads for a late update after the first render, protecting user input
• retains a URL loop guard when session storage is unavailable
• does not change package versions

Verification

• npm run lint
• npx ng test kit --watch=false (53 files, 982 tests)
• npm run prebuild:kit
• git diff --check